Magnetometry based on electron spin defects

ABSTRACT

A magnetometer includes a sample signal device; a reference signal device; a microwave field generator operable to apply a microwave field to the sample signal device and the reference signal device; an optical source configured to emit light including light of a first wavelength that interacts optically with the sample signal device and with the reference signal device; at least one photodetector arranged to detect a sample photoluminescence signal including light of a second wavelength emitted from the sample signal device and a reference photoluminescence signal including light of the second wavelength emitted from the reference signal device, in which the first wavelength is different from the second wavelength; and a magnet arranged adjacent to the sample signal device and the reference signal device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/472,209 filed Sep. 10, 2021, which claims the benefit of U.S.Provisional Patent Application No. 63/076,750, filed on Sep. 10, 2020,the entire contents of which are hereby incorporated by reference.

BACKGROUND

Various sensors are available that rely on classical physical phenomenafor detecting properties such as electric or magnetic fields. In certaincases, magnetic field detectors are limited by one or more of theirsensitivity, dynamic range and/or form factor.

SUMMARY

The present disclosure relates to electron spin defect basedmagnetometry. In some examples, the disclosure describes a magnetometerincluding a sample signal device; a reference signal device; a microwavefield generator operable to apply a microwave field to the sample signaldevice and the reference signal device; an optical source configured toemit light including light of a first wavelength that interactsoptically with the sample signal device and with the reference signaldevice; at least one photodetector arranged to detect a samplephotoluminescence signal including light of a second wavelength emittedfrom the sample signal device and a reference photoluminescence signalincluding light of the second wavelength emitted from the referencesignal device, in which the first wavelength is different from thesecond wavelength; and a magnet arranged adjacent to the sample signaldevice and the reference signal device.

In some implementations, the sample signal device includes a sampleelectron spin defect layer and the reference signal device includes areference electron spin defect layer, each electron spin defect layerincluding a plurality of lattice point defects, and the light of thefirst wavelength excites the plurality of lattice point defects of thesample electron spin defect layer and the reference electron spin defectlayer from ground states to excited states.

Examples of magnetometers may include one or more of the followingfeatures. The sample signal device includes a first substrate on whichthe sample electron spin defect layer is disposed, and the referencesignal device includes a second substrate on which the referenceelectron spin defect layer is disposed. The plurality of lattice pointdefects includes a plurality of nitrogen-vacancy (NV) defects, and thesample electron spin defect layer and the reference electron spin defectlayer each include a diamond layer including carbon 12 and/or carbon 13.The plurality of lattice point defects includes a plurality ofsilicon-carbide (SiC) defects. The sample electron spin defect layer andthe reference electron spin defect layer each has a thickness of betweenabout 1 micron and about 1 mm. The sample electron spin defect layer andthe reference electron spin defect layer each has a thickness of betweenabout 200 microns and about 5 millimeters. The sample electron spindefect layer and the reference electron spin defect layer each hasapproximately the same thickness. The microwave field generator includespatterned layers of metal positioned at one or both of a surface of thesample electron spin defect layer and a surface of the referenceelectron spin defect layer, and an interface of the sample electron spindefect layer with another layer of material and an interface of thereference electron spin defect layer with another layer of material.

Examples of magnetometers may also include one or more of the followingfeatures. The sample signal device is spatially separated from thereference signal device by between about 1 cm and about 10 cm. The atleast one photodetector includes a first photodetector operable todetect the sample photoluminescence signal and a second photodetectoroperable to detect the reference photoluminescence signal. Themagnetometer includes a microprocessor coupled to the at least onephotodetector, the microprocessor configured to cause the at least onephotodetector to detect the sample photoluminescence signal at a firsttime and the reference photoluminescence signal at a second time, inwhich the first time is different from the second time. The magnetometerincludes an optical switch configured to alternately direct the lightemitted by the optical source towards either the sample signal device orthe reference signal device.

Examples of magnetometers may also include one or more of the followingfeatures. The magnetometer includes a microprocessor coupled to the atleast one photodetector and configured to receive light measurementsignals corresponding to the sample photoluminescence signal and to thereference photoluminescence signal from the at least one photodetector,and the microprocessor is configured to analyze the light measurementsignals to determine characteristics of a time-dependent magnetic fieldto which the magnetometer is exposed. The microprocessor is configuredto remove noise from a sample light measurement signal corresponding tothe sample photoluminescence signal using a reference light measurementsignal corresponding to the reference photoluminescence signal. Thesample signal device is arranged closer to a source of thetime-dependent magnetic field than is the reference signal device. Themagnetometer includes a shielding element, the shielding elementarranged to decrease an intensity of the time-dependent magnetic fieldat the reference signal device compared to an intensity of thetime-dependent magnetic field at the sample signal device. Removing thenoise includes subtracting the reference light measurement signal fromthe sample light measurement signal.

Examples of magnetometers may also include one or more of the followingfeatures. The magnet is arranged such that the sample signal device andthe reference signal device are exposed to approximately the samemagnitude of a magnetic field originating at the magnet. Themagnetometer includes an enclosure, and the sample signal device, thereference signal device, the microwave field generator, the opticalsource, the at least one photodetector, and the magnet are contained inthe enclosure. The enclosure is configured to attach to an article ofclothing. The sample signal device is closer to the article of clothingthan is the reference signal device when the magnetometer is attached tothe article of clothing. The enclosure is configured to removably adhereto human skin. The sample signal device is closer to the human skin thanis the reference signal device when the magnetometer is adhered to thehuman skin. The enclosure includes an attachment element operable toattach the enclosure to an object, and the sample signal device iscloser to the attachment element than is the reference signal device.The microwave field generator includes an antenna. The microwave fieldgenerator includes a co-planar waveguide, loop, wire, or coil.

Examples of magnetometers may also include one or more of the followingfeatures. The optical source includes a light emitting diode or a laser.The first wavelength is about 532 nm. The optical source is arranged toemit the light including the first wavelength in a direction towards oneor both of the sample signal device and the reference signal device. Themagnetometer includes at least one optical component arranged betweenthe optical source and the sample signal device. The at least oneoptical component includes at least one of a lens, a mirror, adiffraction grating, and a beam-splitter. A first photodetector of theat least one photodetector is positioned so that a detecting surface ofthe first photodetector faces an area of the sample signal device towhich the light from the optical source is directed. The microwave fieldgenerator is positioned adjacent to an area of the sample signal deviceto which the light from the optical source is directed. The magnetometerincludes at least one lens between the sample signal device and the atleast one photodetector. The magnetometer includes at least one opticalfilter between the sample signal device and the at least onephotodetector. The at least one optical filter is configured to filterout wavelengths of light different than the second wavelength. Themagnet is a permanent magnet.

This disclosure also describes methods. In some examples, the disclosuredescribes a method of measuring a time-varying magnetic field using amagnetometer, the magnetometer including a sample signal device, areference signal device, a microwave field generator, an optical source,at least one photodetector, and a magnet, the method including directinglight from the optical source toward the sample signal device and towardthe reference signal device, in which the light from the optical sourceincludes light of a first wavelength; receiving, at the at least onephotodetector, a sample photoluminescence from the sample signal deviceand a reference photoluminescence from the reference signal device, inwhich the sample photoluminescence and the reference photoluminescenceincludes light of a second wavelength different from the firstwavelength; and determining a sample measurement signal due to thesample photoluminescence and a reference measurement signal due to thereference photoluminescence; and determining information about thetime-varying magnetic field based on the sample measurement signal andthe reference measurement signal.

In some examples, the method includes one or more of the followingfeatures. The sample signal device includes a sample electron spindefect layer and the reference signal device includes a referenceelectron spin defect layer, each electron spin defect layer including aplurality of lattice point defects, and directing the light includesexciting the plurality of lattice point defects of the sample electronspin defect layer and the reference electron spin defect layer fromground states to excited states. Determining the information about thetime-varying magnetic field includes using the reference measurementsignal to remove noise from the signal measurement signal. Removing thenoise includes subtracting the reference measurement signal from thesignal measurement signal. The method includes causing the at least onephotodetector to detect the sample photoluminescence at a first time andthe reference photoluminescence at a second time, in which the firsttime is different from the second time. The method includes positioningthe sample signal device closer to a source of the time-varying magneticfield than is the reference signal device. The source includes a heart.The method includes applying a microwave signal to the sample signaldevice and to the reference signal device using the microwave fieldgenerator.

In some examples, the method also includes one or more of the followingfeatures. The method includes attaching an enclosure containing themagnetometer to an article of clothing such that the sample signaldevice is closer to the clothing than is the reference signal device.The method includes adhering an enclosure containing the magnetometer toskin such that the sample signal device is closer to the skin than isthe reference signal device. The method includes, at a first time,directing the light from the optical source towards the sample signaldevice, and at a second time, different from the first time, directingthe light from the optical source towards the reference signal device.The method includes using the magnet to apply an approximately equalmagnetic field to the sample signal device and to the reference signaldevice.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages of the invention will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic that illustrates an exemplary energy level schemefor a nitrogen-vacancy defect.

FIG. 2 is a plot of exemplary photoluminescence intensity versus appliedmicrowave frequency.

FIG. 3 is a schematic that illustrates an exemplary process forperforming electron spin defect based magnetometry to detect an ACmagnetic field.

FIG. 4 is a schematic that illustrates an example of a device that maybe used to perform electron spin defect based magnetometry.

FIGS. 5-8 are schematics illustrating example magnetometers.

DETAILED DESCRIPTION

The present disclosure relates to electron spin defect basedmagnetometry. In particular, the present disclosure relates totechniques for sensing magnetic fields by monitoring Zeeman shifts ofelectron spin sublevels established by the presence of atomic defects insolid-state lattice structures, and devices for performing the same.

More specifically, electron spin defect based magnetometers includequantum sensors that leverage the occurrence of an electronic spindefect in a solid state lattice, where the spin can be both initializedand read out optically. In certain implementations, the defect may ariseas an atomic-level vacancy in a lattice structure, such as a vacancyoccurring near a nitrogen atom substituted in place of a carbon atomwithin diamond. Accordingly, a single spin defect center, as anatom-scale defect, may be used to detect magnetic fields with nanometerspatial resolution, while an ensemble of uncorrelated spin defects maybe used with spatial resolution given by the ensemble size (e.g., on theorder of microns) typically with an improvement in sensitivity given by√N, where N is the number of spin defects. Moreover, in someimplementations, electron spin defect based magnetometers may exhibitrelatively long coherence times, such as times approaching 1 second ormore. Additionally, electron spin defect based magnetometers may beoperated at room temperature and, in certain cases, within relativelycompact structures, allow for portability and reduction in magnetometercosts, which may be advantageous in health related applications such asmeasuring magnetic fields emanating from the heart.

A brief description of electron spin defect based magnetometry will bedescribed with reference to FIGS. 1-2 and in particular with respect tonitrogen vacancy (NV) magnetometry, though the techniques and devicesdisclosed herein may be applicable to other materials, including othertypes of electron spin defects, as well. An NV center is a defect in adiamond lattice that contains a substitutional nitrogen atom in place ofcarbon, adjacent to a vacancy in the diamond lattice. Thenegatively-charged state of the defect provides a spin triplet groundlevel which can be initialized, coherently manipulated with longcoherence time and readout, using optical means. FIG. 1 is a schematicthat illustrates an energy level scheme 100 for an NV defect. The NVdefect behaves as an artificial atom within the diamond lattice thatexhibits a broadband photoluminescence emission with a zero phonon lineat 1.945 eV or λ_(PL)=637 nm. Moreover, the ground level 102 of the NVdefect is a spin triplet state, having spin sub-levels of the m_(s)=0state 104 and the m_(s)=+/−1 states 106, separated by K=2.87 GHz in theabsence of a magnetic field. The defect can be optically excited to anexcited level 108, which also is a spin triplet having an m_(s)=0 state110 and m_(s)=+/−1 states 112. Once optically excited into the excitedlevel 108, the NV defect can relax primarily through one of twomechanisms: a) through a radiative transition and phonon relaxation,thus producing a broadband red photoluminescence; or b) through asecondary path 114 that involves non-radiative intersystem crossing tosinglet states 116.

The decay path branching ratios from the excited state manifold back tothe ground state manifold depends on its initial spin sublevelprojection. Specifically, if the electron spin began in the m_(s)=+/−1states, there is approximately a 30% chance for the spin to decaynon-radiatively through the secondary path 114, down to the m_(s)=0state. The population of the spin sublevels can be manipulated by theapplication of a resonant microwave field to the diamond. Specifically,at a particular microwave frequency corresponding to the transitionenergy cost between the 0 and +/−1 states, transitions occur betweenthose sublevels, resulting in a change in the level of photoluminescenceof the system. In particular, if the spin is initialized into them_(s)=0 state, and the population is transferred to one of the +/−1states by the resonant microwave drive, the photoluminescence rate uponsubsequent optical illumination will decrease.

In the absence of a magnetic field, this drop in photoluminescence maybe observed by sweeping the microwave frequency, as depicted in thebottom-most photoluminescence (PL) intensity line 202 shown in FIG. 2 ,which is a plot of PL intensity versus applied microwave frequency. Uponapplying a magnetic field in the vicinity of the NV defect, however, thedegeneracy of the m_(s)=+/−1 spin sublevels is lifted by the Zeemaneffect, leading to the appearance of two electron spin resonance (ESR)transitions, corresponding to dips in the PL spectrum (see upper PLlines 204 in FIG. 2 ). The value Δν corresponds to the ESR linewidth,typically on the order of 1 MHz and the value C is the ESR contrast. Todetect small magnetic fields, the NV transitions may be driven at thepoint of maximum slope (see, e.g., 206 in FIG. 2 ). At this point ofmaximum slope, a time-domain change in the photoluminescence may bedetected, from which a time-domain change in magnetic field can bederived. The signal may be expressed ast (∂I₀/∂B)×δB×Δt, where I₀ is theNV defect PL rate, δB is the infinitesimal magnetic field variation, andΔt is the measurement duration, much smaller than the timescale on whichthe magnetic field changes A single NV defect therefore can serve as amagnetic field sensor with an atomic-sized detection volume. To improvesensitivity, a collective response of an ensemble of NV defects may bedetected, such that the collected PL signal is magnified by the number Nof the sensing spins and therefore improves the shot-noise limitedmagnetic field sensitivity by a factor of 1/√N.

Magnetic field sensitivity can further be improved if the magnetic fieldto be measured is periodic in time (e.g., an AC field). The improvementin sensitivity with a classical AC field is a result of a prolongationof the NV spin coherence that can be achieved through dynamicaldecoupling of the central spin from its environment. To avoid broadeningof the ESR linewidth caused by the laser readout process and the drivingmicrowave field, the spin manipulation, spin readout and phaseaccumulation (magnetic field measurement) may be separated in time. Todo so, a series of microwave pulses are applied in sequence to the NVdefect (or defects) that is in a prepared state |0>. Here |0> and |1>denote the electron spin states m_(s)=0 and m_(s)=1. FIG. 3 is aschematic that illustrates an example of electron spin defect basedmagnetometry for an AC magnetic field, in which a microwave pulsesequence is be applied to an NV defect or ensemble of NV defects. Thepulse sequence may also referred to as the “Hahn echo,” though otherdynamical decoupling pulse sequences may be used instead. In particular,a first light pulse 302 is applied to the NV defect, or ensemble of NVdefects, to place them in a prepared state |0>. While the NV defect(s)are exposed to an alternating magnetic field 300, a first π/2 microwavepulse 304, is applied to the NV defect(s) to rotate the electron spin ofthe NV defect(s) from the prepared state |0> to a coherent superposition|ψ>=1/√2*(|0>+e^(iφ)|1>) which evolves over a total free precession time2τ, if the microwave drive Rabi frequency is larger than other terms inthe Hamiltonian, such as NV hyperfine coupling, and the size of themagnetic field to be measured The phase φ set to zero by definition,choosing the microwave drive field to be along the y axis (arbitrary).During the free precession time, the electron spin interacts with theexternal magnetic field. The |1> state acquires a phase with respect tothe |0> state, corresponding to a precession of the spin in the planeperpendicular to the spin quantization axis in a Bloch sphere picture.Then, a first π microwave pulse 306 is applied to “swap” the phaseacquired by the |0> and |1> states. For slow components of theenvironmental magnetic noise, the dephasing acquired during the firsthalf of the sequence is compensated and spin dephasing induced by randomnoise from the environment may be reduced. Additionally, frequencycomponents much higher than the frequency 1/τ average out to zero. Slowcomponents may include, e.g., DC components and low frequency componentson the order of several Hz, several tens of Hz, several hundreds of Hz,and 1-1000 kHz such as 10 Hz or less, 100 Hz or less, or 500 Hz or less,1 kHz or less, 10 kHz or less, 100 kHz or less and 1 MHz or less. Insome implementations, the pulse 306 is applied at the zero crossing ofthe classical AC magnetic field so that the spin phase accumulation dueto the classical AC field can be enhanced. In some implementations,multiple π microwave pulses 306 are applied periodically. After applyingone or more π microwave pulses 306, the phase (p and thus the magneticfield is measured by applying a second π/2 pulse 308 that projects theNV electronic spin back onto the quantization axis. The total phaseaccumulation is thus converted into an electron population, which may beread out optically through the spin-dependent PL of the NV defect(s).That is, a second optical pulse 310 is applied to the NV defect, orensemble of NV defects, resulting in a photoluminescence that is readout by an optical detector. To derive the magnetic field B(t) from thePL measurement, the function describing the evolution of the Sz operatorunder the pulse sequence is multiplied by the noise and signal fields,which is then integrated to get the phase accumulation and subsequentlymultiplied by contrast and total photoluminescence rate to get thephotoluminescence signal (sine magnetometry). For cosine magnetometry,the filter function is convolved with the power spectral density of thenoise and signal fields to get the phase variance, which is thenmultiplied by contrast and photoluminescence rate. Sensitivity comparedto the continuous-wave driving technique may improve by a factor of atleast (T2/T2*)^(1/2), in which T2 is the coherence time of the NV underAC magnetometry and T2* is inversely proportional to the NV linewidth.

As explained above, an NV defect is just one example of a type of spindefect that may be used to perform electron spin defect basedmagnetometry. In other implementations, one or more spin defects may beformed in silicon carbide. SiC defects include defects due to othersubstitutional atoms, such as, e.g. phosphorus, in the SiC lattice.Similar techniques for detecting magnetic fields as described hereinwith NV defects may be employed with the SiC defects.

FIG. 4 is a schematic that illustrates an example of a device 400 thatmay be used to perform electron spin defect based magnetometry, asdescribed herein. Device 400 includes a substrate 402 and an electronspin defect layer 404 formed on the substrate 402. The electron spindefect layer 404 may include multiple lattice point defects, such as NVdefects formed in diamond, as described herein. The defect layer 404containing the NV defects may be formed, in some cases, from up to99.999% carbon 12. In some implementations, carbon 13 may be usedpartially in place of carbon 12. The electron spin defect layer 404 isnot limited to NV defects formed in diamond, which is typicallyelectronic grade, and may include other lattice point defects in othermaterials, such as silicon carbide. The electron spin defect layer 404may be a sub-layer of a larger layer 406 that is without the electronspin defects. For example, larger layer 406 may be a diamond layerwithout NV defects, whereas a top portion of the diamond layercorresponds to the defect layer 404.

The thickness of the defect layer 404 may vary. For example, in someimplementations, the thickness of the defect layer 404 may be greaterthan about 2-3 microns, such as greater than 10 microns, greater than 50microns, greater than 100 microns, greater than 250 microns, greaterthan 500 microns, or greater than 750 microns. The thickness of thedefect layer 404 may be less than about 1 millimeter, such as less than750 microns, less than 500 microns, less than 250 microns, or less than100 microns. Other thicknesses may be used as well. Thickness of layer404 is referenced here as being the distance from the interface betweenlayer 404 and layer 406 and the opposite facing surface of layer 404. Ifthe defect layer is a part of or formed on layer 406, then layer 406 mayhave its own separate thickness. For example, layer 406 may have athickness between about 200 microns and about 5 millimeters. Thicknessof layer 406 is referenced here as being the distance from the interfacebetween layer 404 and layer 406 and the interface between layer 406 andsubstrate 402.

In some implementations, the layer 404 (or the layer 406) is secured tothe substrate using an adhesive including, e.g., epoxies, elastomers,thermoplastics, emulsions, and/or thermosets, among other types ofadhesives. In some implementations, electrical contacts are formedbetween the layer 404 (or the layer 406) and the substrate 402. Forexample, in some cases, the substrate may include a semiconductormaterial, such as silicon, in which one or more circuit elements (416,418, 420) are fabricated. Electrical connections may be formed withinthe substrate 402 to provide an electrical connection between thecircuit elements 416, 418, 420 and one or more components formed in oron layer 404 (or layer 406).

Device 400 further includes a microwave field generator 410 to provide amicrowave field to the electron spin defects of the defect layer 404. Inthe present example shown in FIG. 4 , microwave field generator 410includes a thin film antenna formed on an upper surface of the defectlayer 404. In some implementations, the microwave field generator 410includes a patterned layer of metal on a surface of the defect layer404, within layer 406 or at the interface between defect layer 404 andlayer 406. The microwave field generator 410 may include a co-planarwaveguide, a wire, a loop or a coil of electrically conductive material,such as metal. The microwave field generator 410 may be positionedadjacent to the area of the defect layer 404 to which the light from aspin defect excitation optical source is directed.

In some implementations, the device 404 includes a microwave fieldcontrol circuit 416. The microwave field control circuit 416 may beformed in or on the substrate 402. For example, in some implementations,the control circuit 416 may be a circuit element formed within a siliconsubstrate. The control circuit 416 may be coupled, e.g., directlyelectrically connected, to the microwave field generator 410 to providea microwave source signal to the microwave field generator 410 so thatthe microwave field generator 410 emits a microwave field toward thedefect layer 404. The microwave source signal may optionally be a pulsedmicrowave source signal. In some implementations, a microwave frequencyof the microwave source signal is between about 2 GHz and about 4 Ghz.In some implementations, the microwave field generator 410 emits signalsat multiple frequencies spaced apart from one another to driveadditional energy level splittings. For example, in someimplementations, the microwave field generator 410 may be operated toemit microwave signals that address NV hyperfine transitions. In someimplementations, the microwave control circuit 416 is configured toprovide a control signal that generates a pulsed microwave signal at thegenerator 410. In some implementations, the microwave control circuit416 is configured to provide a control signal that generates acontinuous wave microwave signal at the generator 410.

In some implementations, the device 400 includes a photodetector 412arranged to detect photoluminescence emitted from the electron spindefects of the defect layer 404. The photoluminescence may include oneor more wavelengths of light, such as wavelengths of about 637 nm,corresponding to the emission wavelength of an NV defect. Thephotodetector 412 may be positioned on an upper surface of the defectlayer 404 and in direct contact with the defect layer 404 as shown inFIG. 4 . In some implementations, the photodetector 412 is positioned sothat a detecting surface of the photodetector 412 faces an area of thedefect layer 404 to which the light from an optical source is directed.The photodetector 412 may be secured to the defect layer 404 using anadhesive that is optically transparent to the wavelengths of lightemitted by the NV defects. Alternatively, or in addition, thephotodetector 412 may be formed beneath defect layer 404, such as at aninterface between substrate 402 and layer 404 or within substrate 402.For example, in some implementations, the photodetector may be a siliconbased photodetector formed within the substrate 402. In someimplementations, an optical component is positioned between thephotodetector 412 and the defect layer 404. For example, the opticalcomponent may include one or more of a lens, a beam-splitter, adiffraction grating, an optical filter, and/or a mirror. The opticalfilter may be configured to filter out wavelengths of light differentthan the wavelength of light emitted by the defects of the defect layer404.

In some implementations, the device 404 includes a microprocessor 418,in which the microprocessor 418 is coupled to the photodetector 412 toreceive a light measurement signal from the photodetector and in whichthe microprocessor is configured to analyze the light measurement signalto determine the characteristics of a magnetic field to which the device404 is exposed. The microprocessor 418 may be formed in or on thesubstrate 402. For example, in some implementations, the microprocessor418 may be a circuit element formed within a silicon substrate. Themicroprocessor 418 may be coupled, e.g., directly electricallyconnected, to the photodetector 412. In some implementations, the device400 includes multiple photodetectors, such as a photodiode array. Thephotodetectors 412 may be located at multiple different positions aroundthe defect layer 404 in order to maximize collection of light emitted bythe defect layer 404. Though the microprocessor 418 is depicted as beingformed in the substrate 402, the microprocessor may be located remotelyfrom the magnetometer. For example, in some implementations, themagnetometer may include a transmitter/receiver to wirelessly receivecontrol and analysis signals from the microprocessor 418 and towirelessly transmit feedback and measurement data to the microprocessor.

In some implementations, the device 400 includes an optical source 408configured to emit light. The light emitted by the optical source 408may include a first wavelength that excites the one or more latticepoint defects within the defect layer 404 from a ground state to anexcited state. The first wavelength is different from a secondwavelength that is emitted by the lattice point defects upon relaxation.The first wavelength may be, e.g., about 532 nm to excite NV defects inthe defect layer 404. The optical source 408 may include, e.g., a lightemitting diode, a laser, or a broadband source that includes filtersconfigured to block transmission of wavelengths other than those used toexcite the lattice point defects. The optical source 408 may be arrangedto emit light 422 toward the defect layer 404. For example, the opticalsource 408 may be angled so that light 422 exiting the source 408travels a path toward the defect layer 404. Alternatively, one or moreoptical elements may be positioned in front of the light emitted fromthe source 408 to redirect the light toward the defect layer 404. Forexample, the one or more optical components may include a lens, amirror, a beam splitter, and/or a diffraction grating.

In some implementations, the device 404 includes an optical sourcecircuit, i.e., a driver 420 for the optical source, in which the driver420 is coupled to the optical source 408 to provide a control signal todrive the optical source. The driver 420 may be formed in or on thesubstrate 402. For example, in some implementations, the driver 420 maybe a circuit element formed within a silicon substrate. The driver 420may be coupled, e.g., directly electrically connected, to the opticalsource 408. In some implementations, the microprocessor 418 is coupledto one or both of the microwave field control circuit 416 and the driver420 to control operation of the field control circuit 416 and/or thedriver 420.

In some implementations, the device 400 includes a magnet 414. Themagnet 414 may be arranged adjacent to the electron spin defect layer404. The magnet 414 is provided to induce the Zeeman effect and lift thedegeneracy of the m_(s)=+/−1 spin sublevels. In some implementations,the magnet 414 is a permanent magnet. In some implementations, themagnet 414 is an electromagnet. The magnet 414 may be positioneddirectly on the substrate 402, on layer 406, or on layer 404, amongother locations. The magnet geometry may be chosen to minimize effectsof inhomogeneous broadening between distinct defects in the defect layer406.

In some implementations (e.g., some scalar magnetometryimplementations), the magnet 414 is arranged such that the bias magneticfield generated by the magnet 414 aligns with spin axes of the NVdefects. In some implementations (e.g., some vector magnetometryimplementations), the magnet 414 is arranged so as to split PL intensitylines from the NV defects into four individual lines that togetherindicate all vector components of the magnetic field to be sensed.

In some implementations, during operation of the magnetometer,environmental instabilities such as laser intensity fluctuations,temperature instability, mechanical disturbances (e.g., vibrations),and/or environmental magnetic field interference may affect lightmeasurements by the photodetector. For example, an environmentalmagnetic field may act alongside the time-varying magnetic field that isthe object of detection (referred to herein as the sample signal), suchthat the sample signal cannot be independently detected.

To decrease the effects of these and other noise sources, amagnetometer, in some implementations, is configured to include orproduce a reference (also referred to herein as a baseline) signal thataccounts for noise in the system. In certain implementations, noise mayaffect both the reference signal and the sample signal in the same orsubstantially similar ways, such that a difference between the samplesignal and the reference signal represents, mostly or wholly, magneticfield contributions from the time-varying magnetic field to be detectedfrom the sample.

The magnetometer may be configured to include or produce a referencesignal in multiple different ways. For instance, in someimplementations, the magnetometer (such as magnetometer 400) includes asecond defect layer in addition to the first defect layer, where thesecond defect layer is provided to generate the reference signal. Insome implementations, the magnetometer is constructed so that the seconddefect layer is less sensitive to the sample signal to be detected thanthe first defect layer (e.g., by being positioned further from a fieldsource of the sample signal than is the first defect layer), or is notat all sensitive to the sample signal.

FIG. 5 shows an example schematic of a magnetometer 500 configured toproduce a reference signal and a sample signal. Although magnetometerelements in FIG. 5 are shown schematically as discrete components, amagnetometer configured as in FIG. 5 may be implemented physically asdescribed in reference to FIG. 4 , e.g., using thin-film componentspartially or wholly in contact with one another.

In this example, an optical source 502 directs incident light 504towards a sample signal device 506 and a reference signal device 508. Asdescribed above, the incident light 504 may interact with one or moreoptical components between the optical source 502 and the devices 506,508; in this example, the incident light 504 passes through a focusinglens 510 and is redirected by mirrors 512, 514. The sample signal device506 and the reference signal device 508, which may take a variety offorms, are each sensitive to magnetic fields and interact optically withthe incident light 504 to generate respective photoluminescence 520,522, as described below. The intensities of the respectivephotoluminescence 520, 522 are indicative of respective time-varyingmagnetic fields to which the respective devices 506, 508 may be exposed.

In some implementations, as shown in FIG. 5 , the sample signal device506 and the reference signal device 508 each include a respective defectlayer 507, 509 (e.g., a sample electron spin defect layer 507 and areference electron spin defect layer 509). The defect layers 507, 507are disposed on respective substrates 516, 518 and may have some or allof the characteristics described above in reference to defect layer 404.For example, the defect layers 507, 509 may be sub-layers of respectivethicker layers (not shown in FIG. 5 ). In some implementations, separatedefect layers 507, 509 may be disposed on different portions of onesubstrate.

In some implementations, the defect layers 507, 509 may be configured tohave approximately the same thickness (e.g., within about 1 micron, orwithin about 10 microns, or within about 100 microns). When the defectlayers 507, 509 have approximately the same thickness, photoluminescenceoriginating from the respective defect layers 507, 509 may be of moresimilar intensity, other factors being equal.

The magnetometer 500 is configured such that the sample signal device506 is more sensitive to a time-varying magnetic field 503 emitted froma sample 511 than is the reference signal device 508. As describedabove, photoluminescence from the sample signal device 506 and thereference signal device 508 (e.g., from a sample electron spin defectlayer and a reference electron spin defect layer) can be analyzed todetermine magnetic fields felt by the sample signal device 506 and thereference signal device 508.

For example, in some implementations, as shown in FIG. 5 , the samplesignal device 506 is closer to the sample 511. For example, thereference signal device 508 may be between about 1 cm and about 10 cmfurther away from the sample 511 than the sample signal device 506. Insome implementations, a magnetometer, such as magnetometer 500, includesattachment elements (e.g., straps and/or adhesives) for attachment to orclose to the sample 511, and the devices 506, 508 may be positioned inreference to the attachment elements such that the devices 506, 508 havethe above relative positions when the magnetometer is attached to asubject or object being observed.

In the example of FIG. 5 , the incident light 504 passes through andinteracts with the sample signal device 506 first, and then interactswith the reference signal device 508. In other implementations, incidentlight may first interact with a reference signal device 508 and thenwith a sample signal device 506, or incident light may be split suchthat separate portions of the incident light interact with the samplesignal device 506 and the reference signal device 508, respectively, asdescribed in reference to FIG. 6 below.

Photoluminescence 520, 522 emitted from devices 506, 508 (in thisexample, from electron spin defects of the defect layers 507, 509) isdetected at photodetectors 524, 526. In various implementations, thephotodetectors 524, 526 may be separate components distanced away fromthe respective devices 506, 508, or the photodetectors 524, 526 may bedisposed, for example, on a surface of the defect layers 507, 509, asdescribed above in reference to photodetector 412. The possible relativeconfigurations and arrangements described for the photodetector 412 anddefect layer 404 may also describe one or more photodetectors used in amagnetometer having two defect layers.

In some implementations, one or more optical components may interactwith the photoluminescence before the photoluminescence is detected. Forexample, as shown in FIG. 5 , respective optical filters 528, 530 andlenses 532, 534 are positioned on optical paths between the devices 506,508 and the photodetectors 524, 526. The lenses 532, 534 may beconfigured to focus the photoluminescence 520, 522 into thephotodetectors 524, 526, while the optical filters 528, 530 may beconfigured to block transmission of wavelengths or polarizations otherthan those of the photoluminescence emitted from devices 506, 508optically interacting with the incident light 504 (e.g., thephotoluminescence emitted by the electron spin defects of defectlayers). For example, in some implementations, the optical filters 528,530 are configured to block green light that pumps NV centers in thedefect layers and to not block red light emitted by the NV centers.

The example magnetometer 500 also includes a magnet 536, having some orall of the characteristics described above for magnet 414. The magnet536 may be arranged adjacent to devices 506 and 508, and, in someimplementations, may be configured and/or arranged to provideapproximately the same magnetic field to both devices 506, 508, suchthat variations in the magnetic field are felt equally at the devices506, 508. The devices 506, 508 may be configured to have matchingorientations. For example, in implementations in which the devices 506,508 include respective defect layers 507, 509, the defect layers 507,509 may be arranged to have matching orientations such that the magneticfield supplied by the magnet 536 is aligned with axes of electron spindefects in both defect layers 507, 509; however, in someimplementations, electron spin defects in the two layers do not havealigned spins. As described above, the magnet 536 is provided to inducethe Zeeman effect and lift the degeneracy of the m_(s)=+/−1 spinsublevels.

In some implementations, the example magnetometer 500 additionallyincludes a microwave field generator 538 configured to apply a microwavefield to the devices 506, 508, as described above. The microwave fieldgenerator 538 may include respective thin-film antennas (not shown)formed on upper surfaces of the devices 506, 508, e.g., patterned layersof metal on the respective upper surfaces of respective defect layers507, 509 or at another interface of the defect layers 509, 509. Asdescribed above for microwave field generator 410, the microwave fieldgenerator 538 may include a co-planar waveguide, a wire, a loop or acoil of electrically conductive material, such as metal, or respectivenumbers of these elements separately for the two devices 506, 508. Themicrowave field generator 538 may be positioned adjacent to areas of thedevices 506, 508 to which the incident light 504 is directed. Themicrowave field generator 538 may be configured to apply substantiallythe same wavelength and/or strength of microwave field to the twodevices 506, 508.

Although FIG. 5 shows a single microwave field generator 538, in someimplementations separate microwave field generators may be included todeliver microwaves to separate respective devices emittingphotoluminescence. For example, respective microwave field controlcircuits (not shown) may be formed in or on the substrates 516, 518 andcoupled to respective microwave field generators to provide differentrespective microwave fields to the layers 507, 509. However, in someimplementations, a single microwave field control circuit (formed in oron a substrate 516, 518 or in another location), may provide a matchingfield to both devices 506, 508. For purposes of noise reduction, it maybe beneficial to include one microwave field control circuit and/or onemicrowave field generator providing respective microwave fields to thedevices 506, 508, such that instabilities in microwave field generationand/or transmission are felt equally at the devices 506, 508.

As described above, a microprocessor 540 is coupled to thephotodetectors 524, 526 and is configured to receive signals from thephotodetectors 524, 526 that indicate intensities of photoluminescencereceived at the photodetectors 524, 526. Using the signal received fromthe photodetector 526 (e.g., indicating the photoluminescence emittedfrom the reference signal device 508) as a baseline, the microprocessor540 is configured to remove noise from the sample signal received fromthe photodetector 524. For example, the microprocessor 540 may subtractthe reference signal indicating the reference signal device's 508photoluminescence intensity from the sample signal indicating the samplesignal device's 506 photoluminescence intensity. The reference signalmay be, for example, an electrical signal that is proportional to orotherwise indicative of the optical reference photoluminescenceintensity.

As described above, because noise sources may impact bothphotoluminescence intensities substantially equally, while thetime-sensitive magnetic field has more effect on the sample signaldevice's 506 photoluminescence intensity, the difference between the twophotoluminescence intensities may represent a noise-reduced indicator ofthe time-sensitive magnetic field.

“Removing noise” from a first signal, as used in this disclosure, refersat least to generating a second signal from a first signal, the secondsignal being a noise-reduced version of the first signal. The noise maybe only partially removed in the second signal compared to the firstsignal.

The microprocessor 540 may be configured to perform normalizationprocessing on signals from the photodetectors 524, 526. For example,because, in the example of FIG. 5 , the incident light 504 interactsfirst with the sample signal layer 506, an intensity of incident light504 received at the reference signal device 508 may be decreased (e.g.,by absorption at the sample signal device 506). Therefore, in someimplementations, the microprocessor 540 may be configured to adjust(e.g., multiply by a factor greater than one) intensity signals receivedfrom the photodetector 526 to account for this decrease.

FIG. 6 shows another example of a magnetometer including a referencesignal device and a sample signal device. In this example, amagnetometer 600 includes an optical source 602 emitting incident light604. Optical components including a focusing lens 610, mirrors 611, 612,and a beamsplitter 614 divide the incident light 604 into beams 613, 615that are directed towards a sample signal device 606 and a referencesignal device 608, respectively. In this example, the sample signaldevice 606 includes a sample electron spin defect layer 607 and thereference signal device 608 includes a reference electron spin defectlayer 609. The example magnetometer 600 also includes a lens 628, anoptical filter 632, a magnet 636, and a microwave field generator 638,each having some or all characteristics described for the correspondingelements of FIG. 5 .

In the example of FIG. 6 , respective photoluminescence 622 from each ofthe sample signal device 606 and the reference signal device 608 shareat least part of a common optical path, such that a photodetector 624may detect photoluminescence 622 from both of the devices 606, 608.

In order to differentiate between the two sources of photoluminescence622, in some implementations a microprocessor 640 coupled to thephotodetector 624 may be configured to cause the photodetector 624 toperform time-resolved or time-gated detection. For example,photoluminescence detected at a first time may by emitted by the samplesignal device 606, and photoluminescence detected at a second, differenttime may be emitted by the reference defect device 608. The time-gatingmay be performed based on, for example, different path lengths for lightto and/or from the respective devices 606, 608. For example, theincident light 604 may be encoded (e.g., pulsed), and the detectedphotoluminescence 622 may be analyzed based on the encoding in order todifferentiate between photoluminescence sources.

For example, in some implementations, the incident light 604 may bemodulated by an optical switch. In the magnetometer of FIG. 6 , forexample, beamsplitter 614 may be replaced by an optical switch (e.g., anacousto-optic modulator) that alternately directs the incident light 604either towards device 606 or towards device 608. The optical switch canbe modulated significantly faster than a rate of change of thetime-varying magnetic field 603.

As described in reference to FIG. 5 , the magnetometer 600 is configuredsuch that the sample signal device 606 is more sensitive to atime-varying magnetic field 603 emitted by a sample 611 than is thereference signal device 608. Photoluminescence emitted by the samplesignal device 606 is therefore more strongly indicative of thetime-varying magnetic field than is photoluminescence emitted by thereference defect device 608, while noise sources may affect thephotoluminescences substantially equally. Based on thephotoluminescences as detected by the photodetector 624, themicroprocessor 640 may use the reference photoluminescence as a baseline(e.g., a noise baseline) in order to account for noise in the samplephotoluminescence, and therefore may remove the noise from a signal(e.g., from the photodetector 624) indicative of the samplephotoluminescence.

Although FIG. 6 shows photoluminescence from the sample signal device606 as passing through the reference signal device 608, otherconfigurations are possible and within the scope of this disclosure. Forexample, the sample signal device and the reference signal device mayhave alternative positions and/or orientations, such thatphotoluminescence from each defect layer may be collected by a lenswithout passing through the other device. For example, the sample signaldevice 606 and the reference signal device 608 may be arranged atdifferent respective heights or displaced laterally from one another(and, in some examples, angled differently so as to both be orientedtowards a lens), such that respective photoluminescence emitted by eachdevice 606, 608 does not intersect with the other device on the opticalpaths of the photoluminescence to the photodetector 624. In someimplementations, components of the magnetometer 600 may be configured(e.g., by positioning and/or angling) such that photoluminescenceemitted by each device 606, 608 is focused onto a different respectivesensing portion of the photodetector 624 (e.g., onto different portionsof a charge-coupled device array).

In addition, some implementations may share features of FIG. 5 and FIG.6 . For example, in some implementations, an incident light beam may besplit to separately illuminate both devices (as shown in FIG. 6 ), andphotoluminescence from the devices may be detected by differentrespective photodetectors (as shown in FIG. 5 ).

In some implementations, a magnetometer may include shielding elements(e.g., metal shields) that shield the reference signal device from thesample signal such that sample signal is comparatively weaker at thereference signal device than at the sample signal device. For example,as shown in FIG. 7 , a magnetometer 700 (only partially shown in thisschematic, for simplicity) includes a sample signal device 702 and areference signal device 704, as previously described. The magnetometer700 also includes a metal shield 706 positioned between the referencesignal device 704 and a sample 708 emitting a time-varying magneticfield 710. The arrangement of the metal shield 706 between the referencesignal device 704 and the sample 708 reduces a sensitivity of thereference signal device 704 to the time-varying magnetic field 710, suchthat the reference signal device 704 may provide a more isolatedindication of noise. In some implementations, a material having highmagnetic permeability is used as a shield, e.g., mu-metal.

In some implementations, the components described herein that form themagnetometer, such as the exemplary devices shown in FIGS. 4-6 , may becontained within an enclosure. The enclosure may be formed from amaterial that allows magnetic fields to pass freely to the magnetometerwithin the device, such as plastic. In some implementations, theenclosure is formed from a magnetic shielding material, e.g., mu-metal,and an aperture in the magnetic shielding material, adjacent to thesample signal device 702 (and, in some implementations, the referencesignal device 704) allows the time-varying magnetic field 710 to besensed. In some implementations, the enclosure may be covered partiallyor entirely by a thin thermally conducting layer of material such as,e.g., aerosol, for thermal isolation. In some implementations, theenclosure containing the magnetometer may be configured to attach to anarticle of clothing. In some implementations, the enclosure containingthe magnetometer may be configured as part of a strap, belt, or otherfastener that can be secured to a body. For example, the enclosurecontaining the magnetometer may be secured to a person's chest.Alternatively, or in addition, the magnetometer may be placed in othersstructure that are affixed to a body. In some implementations, theenclosure containing the magnetometer described herein may be configuredto removably adhere to human skin using, e.g., a medically adhesive tapeor other medical adhesive.

When a magnetometer is placed in an enclosure or includes an enclosure,the enclosure and magnetometer may be configured such that a samplesignal device of the magnetometer is closer to a sample that emits ato-be-measured time-varying magnetic field than is a reference signaldevice of the magnetometer. For example, in the example of FIG. 8(which, for simplicity, omits several features shown in FIGS. 5-6 ), asignal sensing device 802 and a reference signal device 804 arecontained within an enclosure 806. The enclosure 806 includes anattachment element 812 (e.g., an adhesive or a clip) configured toattach the enclosure 806 to an object 814 that includes a sample 808that is a source of a time-varying magnetic field 810. The enclosure806, the signal sensing device 802, and the reference signal device 804are relatively configured and arranged such that, when the enclosure 806is attached to the object 814, the sample signal device is closer to thesample 808 that is the reference signal device 804. For example, in someimplementations the sample signal device 802 is closer to a body towhich the enclosure 806 is attached (e.g., closer to clothing to whichthe magnetometer is attached), or is closer to a portion of the body(e.g., closer to a user's heart). In some implementations, when themagnetometer is configured to removably adhere to human skin, themagnetometer and the enclosure are configured such that the samplesignal device is closer to the skin. In some implementations, the samplesignal device 802 is arranged closer to the attachment element 812 thanis the reference signal device 804, which may result in the relativepositions described.

The electron spin defect based magnetometry techniques and devicesdescribed herein are viable for compact, room temperature magnetometry,and are robust to large magnetic field variations. In someimplementations, the magnetometer can be used in applications such asmagnetocardiography to detect magnetic fields from the heart. Inparticular, compact, robust spin defect based magnetometers may be usedto detect magnetic fields emanating from the heart for continuous,long-term monitoring and early detection of various cardiac conditions.

Cardiovascular disease is the number one cause of death worldwide.Electric and magnetic fields generated by the heart contain informationabout the onset of a dangerous condition such as a heart attack orarrhythmia. However, technologies for monitoring this vital organ may bebulky, noisy, and in non-clinical settings can only be used for up to afew days at a time, making the continuous acquisition of data over atbest problematic. Moreover, current analyses must be performed by amedical professional after the data is taken, severely limiting theamount of data that can be analyzed and further increasing the cost (anddecreasing the scope and accessibility) of these vital services.

The sensors required to detect the small magnetic fields tend to requireoperation in a shielded room (such as optically pumped magnetometers),or at cold temperatures (such as SQUIDS), making continuous acquisitionand monitoring difficult. The magnetometers disclosed herein may beused, in certain implementations, as quantum sensors to measure magneticfields from the heart and may be operated outside of a shielded room, atroom temperature and offer a large dynamic range of up to 100 mT.Moreover, the device may be constructed so it is compact and can be worncomfortably and close to the body. The device may configured such that,when the device is being worn and in position on or close to the body, asample signal device is closer to the heart than is a reference signaldevice.

The magnetometers described herein may also be used in applicationsbesides magnetocardiography. For example, the magnetometers may be usedto measure neuron activity. In some cases, the magnetometers may be usedto detect magnetic fields created by electrical currents on a chip,thereby directly mapping on-chip circuit activity. The magnetometersdescribed in this disclosure may be used in any application in whichhigh-sensitive magnetic field measurement is desired.

Embodiments and functional operations described in this specification,such as the operations and analysis performed by the microprocessor, themicrowave control circuit, and the optical source driver, may beimplemented in digital electronic circuitry, or in computer software,firmware, or hardware, including the structures disclosed in thisspecification and their structural equivalents, or in combinations ofone or more of them. Embodiments may be implemented as one or morecomputer program products, i.e., one or more modules of non-transientcomputer program instructions encoded on a non-transient computerreadable medium for execution by, or to control the operation of, dataprocessing apparatus. The computer readable medium may be amachine-readable storage device, a machine-readable storage substrate, amemory device, a composition of matter effecting a machine-readablepropagated signal, or a combination of one or more of them.

The term “data processing apparatus” encompasses all apparatus, devices,and machines for processing data, including by way of example aprogrammable processor, a computer, or multiple processors or computers.The apparatus may include, in addition to hardware, code that creates anexecution environment for the computer program in question, e.g., codethat constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, or a combination of one or moreof them. A propagated signal is an artificially generated signal, e.g.,a machine-generated electrical, optical, or electromagnetic signal thatis generated to encode information for transmission to suitable receiverapparatus.

A computer program (also known as a program, software, softwareapplication, script, or code) may be written in any form of programminglanguage, including compiled or interpreted languages, and it may bedeployed in any form, including as a stand-alone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program does not necessarily correspond to afile in a file system. A program may be stored in a portion of a filethat holds other programs or data (e.g., one or more scripts stored in amarkup language document), in a single file dedicated to the program inquestion, or in multiple coordinated files (e.g., files that store oneor more modules, sub programs, or portions of code). A computer programmay be deployed to be executed on one computer or on multiple computersthat are located at one site or distributed across multiple sites andinterconnected by a communication network.

The processes and logic flows described in this specification may beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows may also be performedby, and apparatus may also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of the disclosure or of what maybe claimed, but rather as descriptions of features specific toparticular embodiments. Certain features that are described in thisspecification in the context of separate embodiments may also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment mayalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination may in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems maygenerally be integrated together in a single software product orpackaged into multiple software products

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. Accordingly, other embodimentsare within the scope of the following claims.

What is claimed is:
 1. A magnetometer comprising: a sample signaldevice; a reference signal device; a microwave field generator arrangedto apply a microwave field to the sample signal device and the referencesignal device; an optical source configured to emit light comprisinglight of a first wavelength that interacts optically with the samplesignal device and with the reference signal device; at least onephotodetector arranged to detect a sample photoluminescence signalcomprising light of a second wavelength emitted from the sample signaldevice and a reference photoluminescence signal comprising light of thesecond wavelength emitted from the reference signal device, wherein thefirst wavelength is different from the second wavelength, and whereinthe light of the second wavelength emitted from the sample signal deviceand the light of the second wavelength emitted from the reference signaldevice share at least part of a common optical path to the at least onephotodetector; and a magnet arranged adjacent to the sample signaldevice and the reference signal device.
 2. The magnetometer of claim 1,comprising a computer system coupled to the at least one photodetectorand configured to receive light measurement signals corresponding to thesample photoluminescence signal and to the reference photoluminescencesignal from the at least one photodetector, wherein the computer systemis configured to differentiate between the light measurement signalscorresponding to the sample photoluminescence signal and the lightmeasurement signals corresponding to the reference photoluminescencesignal by applying time-gated detection, and wherein the time-gateddetection is based on different optical path lengths for (i) the lightof the second wavelength emitted from the sample signal device and (ii)the light of the second wavelength emitted from the reference signaldevice.
 3. The magnetometer of claim 1, comprising a beamsplitterarranged to divide the light emitted by the optical source into twobeams that are directed to the sample signal device and the referencesignal device, respectively.
 4. The magnetometer of claim 1, comprisingan optical switch configured to alternately direct the light emitted bythe optical source towards either the sample signal device or thereference signal device.
 5. The magnetometer of claim 4, wherein theoptical switch comprise an acousto-optic modulator.
 6. The magnetometerof claim 4, wherein the optical switch is configured to switch at a ratefaster than a rate of change of a time-varying magnetic field to whichthe magnetometer is exposed.
 7. The magnetometer of claim 1, comprisingan optical filter arranged on the common optical path to the at leastone photodetector, the optical filter configured to filter light of atleast one wavelength out of the light of the second wavelength emittedfrom the sample signal device and the light of the second wavelengthemitted from the reference signal device.
 8. The magnetometer of claim1, comprising an enclosure, wherein the sample signal device, thereference signal device, the microwave field generator, the opticalsource, the at least one photodetector, and the magnet are arrangedwithin the enclosure, wherein the enclosure is configured to attach toan article of clothing, and wherein the enclosure is configured suchthat the sample signal device is closer to the article of clothing thanis the reference signal device when the magnetometer is attached to thearticle of clothing.
 9. The magnetometer of claim 1, comprising anenclosure, wherein the sample signal device, the reference signaldevice, the microwave field generator, the optical source, the at leastone photodetector, and the magnet are arranged within the enclosure,wherein the enclosure is configured to removably adhere to human skin,and wherein the enclosure is configured such that the sample signaldevice is closer to the human skin than is the reference signal devicewhen the magnetometer is adhered to the human skin.
 10. The magnetometerof claim 1, wherein the at least one photodetector comprises aphotodetector arranged to detect the sample photoluminescence signal andthe reference photoluminescence signal.
 11. A magnetometer comprising: asample signal device; a reference signal device; a microwave fieldgenerator arranged to apply a microwave field to the sample signaldevice and the reference signal device; an optical source configured toemit light comprising light of a first wavelength that interactsoptically with the sample signal device and with the reference signaldevice; at least one photodetector arranged to detect a samplephotoluminescence signal comprising light of a second wavelength emittedfrom the sample signal device and a reference photoluminescence signalcomprising light of the second wavelength emitted from the referencesignal device, wherein the first wavelength is different from the secondwavelength; a magnet arranged adjacent to the sample signal device andthe reference signal device; and a computer system coupled to the atleast one photodetector and configured to receive light measurementsignals corresponding to the sample photoluminescence signal and to thereference photoluminescence signal from the at least one photodetector,wherein the computer system is configured to differentiate between thelight measurement signals corresponding to the sample photoluminescencesignal and the light measurement signals corresponding to the referencephotoluminescence signal by applying time-gated detection, and whereinthe time-gated detection is based on different optical path lengths for(i) the light from the optical source directed to the sample signaldevice and (ii) the light from the optical source directed to thereference signal device.
 12. The magnetometer of claim 11, wherein theoptical source is configured to emit the light of the first wavelengthas encoded light, and wherein the computer system is configured todifferentiate between the light measurement signals corresponding to thesample photoluminescence signal and the light measurement signalscorresponding to the reference photoluminescence signal based on anencoding of the encoded light.
 13. The magnetometer of claim 12, whereinthe encoded light comprises pulsed light.
 14. The magnetometer of claim11, comprising an optical switch configured to alternately direct thelight emitted by the optical source towards either the sample signaldevice or the reference signal device.
 15. The magnetometer of claim 14,wherein the optical switch comprise an acousto-optic modulator.
 16. Themagnetometer of claim 14, wherein the optical switch is configured toswitch at a rate faster than a rate of change of a time-varying magneticfield to which the magnetometer is exposed.
 17. The magnetometer ofclaim 11, wherein the computer system is configured to cause the atleast one photodetector to detect the sample photoluminescence signal ata first time and the reference photoluminescence signal at a secondtime, wherein the first time is different from the second time.
 18. Themagnetometer of claim 17, wherein the magnet is arranged such that thesample signal device and the reference signal device are exposed toapproximately the same magnitude of a magnetic field originating at themagnet.
 19. The magnetometer of claim 11, comprising: a beamsplitterarranged to receive the light emitted by the optical source and splitthe light emitted by the optical source into a first portion and asecond portion, wherein the first portion is directed to the referencesignal device; and a mirror arranged to receive the second portion anddirect the second portion to the sample signal device.
 20. Themagnetometer of claim 11, wherein a first optical path of the light fromthe optical source to the sample signal device has a common portion witha second optical path of the light from the optical source to thereference signal device.